And don't they also charge $4.99/month for DVR service?
In comparison Dish Network sells the 921 for a lot less than that ($549?, slightly less through some other outlets), and I believe they are apparently going to charge an up-front $250 plus rental costs for the upcoming 942. I'm unsure about a retail price for the 942 but it is rumored to be about $600 (not sure about a monthly service fee).
As many other folks point out a lot of cable providers are offering HD-DVRs for more like $10/month (without an upfront fee for installation in many cases). I don't think there are any "purchase" options with cable DVRs, but that will be changing when TiVo releases their announced cable-card compatible HD devices.
With such a wide price range in the market-place I would think pretty tough to guestimate Voom's pricing structure.